Shrink films are typically polymeric films that are applied over or around a substrate. Two portions of the film are bonded together to form a seal or seam that results in a sleeve, envelope, or tube configuration. When heated, the shrink wrap film contracts or shrinks by about 20% to about 70%, and more. The film shrinks to conform to the contours of the underlying substrate. Shrink films typically comprise polyvinyl chloride (“PVC”), glycol modified polyethylene terephthalate (“PETG”), or oriented polystyrene (“OPS”).
The integrity of the shrink film seam needs to be maintained during the shrinking of the film around the container. If not maintained, the seam can partially or totally fail, causing unsightly seams or even seam failure.
There are several different strategies in forming the seam in shrink wrap films, all of which suffer from various drawbacks. The most common seam forming method utilizes a solvent to form the seam in the polymeric film. The solvent is normally applied to the polymeric shrink film immediately prior to forming the seam. The solvent rapidly dissolves the polymeric film and the film welds to itself when peripheral portions of the film are pressed together. Although solvent formed seams provide relatively good tack between peripheral film portions to maintain the seam during shrinking of the film, there are a number of drawbacks. Typical solvents are volatile organic compounds (“VOC” or “VOCs”) that pose potential health and environmental concerns and are subject to environmental regulations. A typical solvent used in this sealing method is tetrahydrofuran (“THF”). THF is used to dissolve PVC films. THF is a VOC, and thus its use has potential disadvantages as mentioned. Other common solvents used in this method have similar difficulties.
Another problem with solvent seaming is that the amount of solvent applied to a seam must be closely regulated. Excess solvent can migrate away from the seam site into other locations on the film. This can cause the film to stick together outside the desired seam site. Additionally, the solvent can potentially diffuse through the film and come into contact with the substrate. Often, shrink wrap films are applied to polyethylene terephthalate (“PET”) containers. THF is reactive with PET containers and tends to degrade such containers. Conversely, a deficient amount of solvent will form an inadequate seal, causing the seam to separate during film shrinking. Application of solvent to the seam site therefore has to be precisely controlled and monitored, and the seals need to be tested; all adding to production costs.
A further drawback of solvent seaming is that solvents are often specific to certain polymers used in the construction of the films. As film construction and film composition change with technology advancements, solvents must be continually tailored to dissolve those films. This causes added cost in research and development; making solvent systems less profitable.
Container recycling efforts are also stymied by solvent seaming methods. Traditional recycling methods often utilize a continuous water flotation process as a means to separate different types of plastic. When PVC shrink labels are applied to PET containers, the flotation separation process can not be used. This is because PVC films and PET containers have densities greater than water, causing both labels and containers to sink during the continuous water flotation process. Because of their high densities, PVC labels will sink with, rather than separate from, PET containers. Floating is thus not a viable means to separate PVC labels from PET bottles. Other, more expensive means must be used to separate the high density labels from the containers.
Another difficulty in solvent seaming is the mechanism used to make the seal, i.e. the dissolving of the polymeric film. Two issues, in particular, arise from the polymeric film being dissolved. One is that the solvent must be applied immediately prior to the seam being formed. The dissolved polymeric film is only capable of bonding to itself for a short time, i.e. a few seconds. If the film is left unsealed, the solvent will quickly evaporate away from the seam site, thus preventing a seam from being formed. The second issue is that no handling or processing can take place between the solvent application and seam formation. The seam must be formed directly after the solvent is applied. If not immediately sealed, the dissolved portion of the film could interfere with converting equipment by sticking, coating, or otherwise leaving a residue on the equipment.
Another seam forming method involves the use of heat-activated or ultraviolet light-activated (“UV”) adhesives. Unlike solvent seaming, these adhesives can be used with low density polyolefin shrink films; thus allowing water separation from dense substrates for recycling efforts. These activatable adhesives are also not initially tacky when deposited on a shrink film; thus allowing further production processes between adhesive application and seam formation. Activatable adhesives require an external initiator to trigger their bonding capabilities. A major difficulty with activatable adhesives is that they typically do not provide a strong bond at the seam. To avoid seam failure, only shrink films with small shrink percentages can be used with activatable adhesives. Thus, in this type of application, only films with shrink percentages of less than 30% are typically used. Limiting films to less than 30% shrinkage limits the possible shape and contour variations for a substrate. With the container industry heading toward the use of highly-contoured, less conventionally-shaped containers, the bond strength provided by activatable adhesives typically does not allow sufficient shrinkage of the films. Additionally, activatable adhesives are slow to activate to a tacky state. This undesirably causes a time-lag in the labeling process and adds to production costs.
Another seam forming method is heat welding. This method uses a heat source (laser, hot bar, etc.) to melt a portion of the polymeric film, and weld the film to itself when the melted portion resolidifies. As with solvent seam forming methods, heat welding suffers from the difficulty that the heat must be applied to the film contemporaneous to the seam being formed. Another disadvantage of heat welding is the energy requirement to bring the polymeric shrink film up to a melting temperature; adding to production costs. The temperature variations of the heat welding apparatus and the amount of time the heat source is in communication with the film must also be closely regulated. Burn-through in the film is a common problem associated with heat welding methods. Abnormalities in the seam associated with burn-through can produce poor seam aesthetics, flaws in the structural integrity of the seam, and other problems. Furthermore, the seams formed by this method often display a ridge or other noticeable deformation in the film that is not aesthetically pleasing.
Another seam forming method employs hot melt adhesives. Problems associated with typical hot melt adhesive systems include the difficulty that the hot melt adhesive must be applied to the film just prior to the seam being formed. Hot melt adhesives are tacky only for a few seconds while they remain melted. Forming the seam must be accomplished before the hot melt adhesive resolidifies. Quick seam formation is required to avoid subsequent film processing problems, discussed previously herein, such as adhesive contamination on production equipment.
Pressure sensitive adhesives (“PSAs”) have also been used to form the seam in shrink wrap films. Like hot melt adhesives, PSAs are applied to the film just prior to the seam being formed. Immediate seam formation is usually required to avoid problems associated with subsequent processing steps. If the seam is not immediately formed subsequent to adhesive application to the film, a release liner is normally used. Typical release liners are paper or polymeric structures that utilize silicon-containing and fluorine-containing materials as a release agent for shrink films. The silicon-containing or fluorine-containing component of the release liner contacts the adhesive layer. The silicon-containing or fluorine-containing material allows the release liners to separate easily from the adhesive when the shrink films are needed for use. The silicon-containing or fluorine-containing material does not strongly adhere to the adhesive and thus provides a releasable interface with the underlying adhesive while providing protection from contamination. The liners are not recyclable due to the presence of silicon and fluorine material, thus raising environmental concerns. Such release liners are suitable only for the landfill and contribute significantly to stress on the environment. Furthermore, occasional transfer to the film or substrate of material containing a silicon or fluorine component can interfere with proper adhesion at the seam, cause problems for subsequent printing operations, and other similar difficulties.
Therefore, a need exists for shrink wrap films and related methods that do not incorporate a solvent in the seaming process. A need also exists for shrink wrap films and related methods that do not incorporate a silicon-containing or fluorine-containing material as a release agent. A need also exists for shrink wrap films and related methods that are useful with low density films. Further, a need exists for shrink wrap films and related methods that leave an aesthetically pleasing seam.